Signals transmitted over long-distance fiber-optic communication systems may be severely degraded by excessive optical noise, which is introduced by optical amplifiers employed to boost signal power throughout each system. The quality of a transmitted optical signal, therefore, is frequently characterized by the optical signal-to-noise ratio (OSNR), which defines the ratio of the signal power carrying the desired information signal and the optical noise added in the communication system. In communication systems without tight optical filtering, the OSNR may be readily determined by spectral analysis of the transmitted signals, which measures the optical power of the information carrying signal as well as the spectral density of the Gaussian noise introduced by the optical amplifiers. Typically, the optical noise appears as a floor in the analyzed optical spectrum, and thus, may be readily measured at optical frequencies, where no optical information signal is transmitted.
In modern optical transmission systems with wavelength multiplexing, the various transmitted signals may be closely spaced in optical frequency, thus making it very difficult to measure the optical noise floor between adjacent signals in the received optical spectrum. In addition, the signals may be passed through narrow-band optical filters that substantially reduce the optical noise floor at frequency components, at which no information carrying signals are transmitted.
A polarization-nulling technique, which substantially removes the polarized optical information signal from the received optical signal, thus revealing the floor of the unpolarized optical noise in the optical spectrum, has been disclosed in “Optical signal-to-Noise Ratio Measurement in WDM Networks Using Polarization Extinction” by M. Rasztovits-Wiech et al., European Conference on Optical Communication, 20-24 Sep. 1998, Madrid Spain, pp. 549-550, and in U.S. Pat. No. 6,813,021 issued Nov. 2, 2004 to Chung et al, U.S. Pat. No. 7,106,443 issued Sep. 12, 2006 to Wein et al, and U.S. Pat. No. 7,149,428 issued Dec. 12, 2006 to Chung et al, which are incorporated herein by reference. The disclosed technique enables measurement of the OSNR within the bandwidth of the transmitted optical information signal, i.e. “in-band OSNR measurement”, when the signal exhibits a substantially constant polarization state.
However, it is well known to those skilled in the art that the output polarization state of a signal transmitted over an optical fiber may fluctuate randomly with time, because standard optical fibers do not maintain the state of polarization of the launched signals. The speed and magnitude of the polarization fluctuations introduced in the fiber depend on the physical environment to which the fiber is exposed, and therefore, may be potentially large. Consequently, these random polarization fluctuations may severely limit in-band OSNR measurements using the polarization-extinction method or other types of polarization analysis.
According to conventional systems, in-band OSNR measurements are usually performed with a measurement apparatus 1, which comprises a tunable optical filter or spectrum analyzer 2, which is connected to a fixed or variable optical polarization state analyzer 3, as shown schematically in FIG. 1(a). An array of photo-detectors 4 is optically coupled to the outputs of the polarization state analyzer 3, from which the OSNR can be measured. The apparatus 1 is optically coupled to the transmission fiber 5 of an optical network. It is appreciated by those skilled in the art that the tunable optical filter 2 may either precede or follow the polarization state analyzer 3 without affecting the overall functionality of the apparatus 1.
In an alternate system illustrated in FIG. 1(b), a measurement apparatus 10 includes a polarization state analyzer 3′ comprised of a variable optical polarization controller 6, with a scan sequencer 7, and a fixed polarization filter or splitter 8, wherein the polarization filter/splitter 8 follows the polarization controller 6. In this embodiment, the tunable filter/spectrum analyzer 2 may either be connected to the output of the polarization filter/splitter 8, as shown in FIG. 1(b), or it may be placed between the polarization controller 6 and the polarization filter 8. Alternatively, it may even precede the polarization controller 6. It is appreciated by those skilled in the art that the preferred arrangement of these three elements depends on the specific details of the optical transmission characteristic of the various elements and components.
In the system illustrated in FIG. 1(b), the polarization controller 6 is adjusted in a predetermined way by the scan sequencer 7 to transform the polarization state that is passed by the polarization filter 8 sequentially into a predetermined, incrementally, continuously varying sequence of optical input polarization states, which substantially cover the entire Poincaré sphere. An optical detector array (not shown) after the spectrum analyzer 2 then records the optical power levels of all probed polarization states at the desired optical frequency components. The signal and noise levels of the analyzed signal are determined from the maximal and minimal values of the power readings recorded for the various probed polarization states, whereby it is assumed that the power level is minimal when the polarized information signal is substantially blocked (or “nulled”) by the polarization filter/splitter 8 and only unpolarized noise is passed to the optical detector. Likewise, it is assumed that the power level is maximal when the polarization state of the information signal is substantially identical to the polarization state analyzed by the polarization filter 8, in which case the entire signal and the noise are both passed to the optical detector. The OSNR in the received signal may then be estimated from a simple analysis of the measured minimal and maximal power levels, as described, for example, in the above referenced U.S. Pat. No. 7,149,428 or in United States Patent Application Publication US 2006/0051087 published Mar. 9, 2006 to Martin et al, entitled “Method for Determining the Signal-to-Noise Ratio of an Optical Signal”.
For the above-described analysis of the polarization characteristics of the received optical information signal, the polarization state of the optical information signal must be substantially constant over the time period needed to cycle the polarization controller 6 through the desired sequence of polarization transformations, including the time needed to measure the optical power levels at the detector array. If the input polarization state of the optical information signal changes substantially during the time period of the OSNR measurement, the polarization controller 6 may not be able to transform the polarization state of the information signal into the two desired polarization states, i.e. the one that is substantially blocked by the polarization filter 8 and the one that is passed through the filter 8 with minimal attenuation. If none of the various polarizations states generated by the optical polarization controller 6 comes sufficiently close to both of these two states, then the OSNR estimated from the measured maximal and minimal power levels may be substantially different from the OSNR present in the received signal. As a result, the estimated OSNR is usually smaller than the true OSNR in the signal. Therefore, polarization fluctuations in the optical information signal may severely degrade in-band OSNR measurements that are obtained by polarization analysis.
An object of the present invention is to overcome the shortcomings of the prior art by providing a simple but effective method to substantially mitigate potentially severe degradations of the polarization extinction in in-band OSNR measurements that are caused by polarization fluctuations in the optical signal to be measured.